There are a number of circumstances in which it is desirable to have the facility to control the level of illumination in a given space by selectively turning off some of the sources of overhead illumination for that space. Thus for example, it may be desirable, and often is a requirement of local ordinance, that the level of illumination in office buildings be reduced for the period at night when the offices are not occupied. It may be desirable to increase or decrease the number of lamps that are illuminated at certain times of the day or in connection with particular activities in a computing room, for example, to reduce glare on monitor tube faces.
It is relatively easy to incorporate provisions for accomplishing the required result when constructing a building. It is not so easy, and may be very expensive, to add that capability to an existing building. It is now almost universal practice to use ceiling mounted fluorescent lamps for space illumination in commercial and industrial buildings and, while the invention is not limited to such applications, to control the level of such illumination is an object of the invention. Several approaches to control have been employed in the past. One is to install a separate wall switch and power wiring for each lamp. Another is to add reactive impedance in the energizing circuit of the fluorescent tube and to provide a means for selectively short circuiting the added impedance with separate control wiring or with a toggle switch which toggles in response to actuation of a wall switch. Still another method is to install, at the lamp, a toggle switch that is actuated by pulling on a string that extends down to within reach.